By Tim Raneyâ€¦Bald Engineer Guy with Glasses
The purpose of this experiment was to demonstrate the magnetocrystalline anisotropic ferrimagnetism sometimes shown by the iron sulfide mineral known as pyrrhotite. Magnetic anisotropy describes an effect where the magnetic properties of ferro- or ferrimagnetic materials show a preferred direction. I was interested in exploring this type of magnetic anisotropy since it occurs naturally based on the mineral’s chemical composition and associated crystal structure.
A similar incarnation of this demonstration experiment appears in Meiners, 1970. Since I bought this two volume set years ago, I have wanted to try this demonstration. I also wanted to craft an experiment with a real hypothesis and maybe build impressive-looking apparatus too. This experiment appeared simple to execute. I just needed to buy a kilogram of pyrrhotite â€“ more is better, right? Hang the specimen from a string near a magnet. I used nice, 100% nylon string too. Lastly, see what happens and record the stellar results and the extraordinary scientific insights. Well, as you might have expected, the experiment failed for a number of reasons. However, the insights gained through further study will assist me in refining the experiment and build better apparatus for future work. The apparatus might look really cool too, but thatâ€™s not the point, right?
The properties of magnetic materials (elements or compounds) can vary depending on the direction and magnitude of an applied magnetic field. For example, a given ferrimagnetic crystal is more easily magnetized along one crystallographic axis (easy direction) compared to another axis (hard direction). The difference in energy for these two states is the anisotropy energy. There are several forms of anisotropy. My focus was on magnetocrystalline anisotropy â€“ an intrinsic property of naturally occurring ferrimagnetic materials due to their crystal structure and independent of their mineral grain size or shape. Anisotropism occurs in both ferro- and ferrimagnetic materials. Familiar examples of ferromagnetic materials include iron, nickel, cobalt and their alloys. Perhaps â€œferrimagnetic materialsâ€ might not seem as familiar, but common examples include the mineral magnetite (Fe3O4), pyrrhotite (Fe1-xS) and ferrites (metal oxide + Fe2O3), a class of metallic oxides. Thus, for the purpose this paper serves, we will digress here and discuss the difference between these two major classes of magnetic materials.
Ferromagnetism is an intrinsic property of iron, certain other elements, alloys and compounds as noted above. Some of the magnetic dipole moments are inherently aligned in these materials. An external magnetic field (Bext) can then align the magnetic moments further, producing a strong magnetic field. Moreover, this induced field can persist to some degree after removing Bext and results in a large net magnetization. This ferromagnetic behavior is due to a quantum physical effect called â€œexchange couplingâ€ whereby the electron spins of one atom interact with other atoms nearby. This effect aligns the atomâ€™s magnetic dipole moments despite the normal random atomic collisions, thus giving ferromagnetic materials their permanent magnetism.
Ferrimagnetic materials, often associated with ionic compounds, e.g., oxides, exhibit more complex forms of magnetic ordering due to their crystal structure. The simple diagram at right shows the magnetic dipole moments in a ferrimagnetic material represented by an oxide. Its structure has two magnetic sub-lattices (A and B) separated by oxygen ions. In this case, the oxygen anions mediate the atomic exchange forces/coupling in contrast to ferromagnetism. END OF PART ONE.
 H.F. Meiners (Ed.), Heat, Electricity & Magnetism, Optics, Atomic & Nuclear Physics, vol. II of Physics Demonstration Experiments, The Ronald Press Company, New York, 1970, pg. 969, sect. 32-3.12 and fig. 32-30.
 R.S. Elliot, Electromagnetics, McGraw-Hill Book Company, Inc., New York, 1966, pp. 443-444.
 B. M. Moskowitz, Hitchhiker’s Guide to Magnetism, Institute for Rock Magnetism, University of Minnesota, College of Science & Engineering, (http://www.irm.umn.edu/IRM/index.html). Downloaded 08 March 2012, pg. 17.
 R.S. Elliot, pg. 451.
 L.R. Moskowitz, Permanent Magnet Design and Application Handbook, Cahners Books International, Inc., Boston, MA, 1976, pg. 26.
 D. Halliday, R. Resnick and J. Walker, Fundamentals of Physics (6th Ed.), John Wiley & Sons, Inc., Hoboken, NJ 2003, pp. 752 and 755.
 B. M. Moskowitz, pg. 11.